This invention relates to an apparatus for detecting subsurface accumulations of gases, for example helium, which may be indicative of nearby hydrocarbon deposits or deposits of radioactive ores. In another aspect, the invention relates to a tip for use on such an apparatus.
In order to take a subsurface gas sample which is representative of the gases present, it is necessary to prevent atmospheric air from contaminating the subsurface sample. This is accomplished by driving a shaft into the earth, the dimensions of the shaft being such that it fills the cavity made as it is driven into the earth. A gas seal formed by the exterior wall of the shaft and the surrounding earth prevents contamination of the sample by atmospheric air. After the shaft has been driven to the desired depth, an aperture near the lower end of the shaft is exposed and the gas sample taken.
The design of the tip of the gas probe is important. If the largest cross-sectional dimension of the tip is greater than the largest cross-sectional dimension of the probe, the probe will not fill the cavity made in the earth by the tip and atmospheric air will contaminate any gas samples collected. For that reason, the tip should not have a greater diameter than the shaft of the probe. In devices wherein the shaft is partially unscrewed from the tip to expose the sampling aperture, the tip must be designed so as to resist rotation in the earth as the shaft is being partially unscrewed therefrom. In the past this was accomplished by utilization of fins on the tip or a wedge-shaped tip. Fin-like protrusions on the tip as in U.S. Pat. No. 3,198,265 caused excessive disturbance of the soil around the shaft and at least partially destroyed the gas seal between the soil and the shaft. The fins also sometimes made removal of the shaft from the earth extremely difficult. Wedge-shaped tips as in U.S. Pat. No. 3,835,710 are difficult to drive into hard earth or earth containing rocks. This is due to the relatively large area of the leading edge of the wedge. The poor penetration of the wedge- or chisel-shaped point causes excessive stresses in apparatuses which utilize these points and frequently results in premature parts failure.